This project is an investigation of the composition and biologic activities of the gastrointestinal flora of mice. The objectives are: (1) Isolation and enumeration of the autochthonous bacteria in specific areas of the gastrointestinal tract with particular emphasis on the strict anaerobic bacteria. 2. Bacterization of germ-free mice with pure cultures of these bacteria to determine the most effective flora a) for controlling or preventing the establishment of foreign bacteria in the gut, b) to allow optimal weight gain and development, c) for optimal utilization of foodstuffs by the host, d) using in vitro systems to delineate some of these mechanisms. 3. Alteration of the gut flora of pathogen free animals with selected antimicrobial agents to achieve a simplified anaerobic flora which will be optimal for the host's growth, resistance to infection, and nutritional demands.